MDCT is the mainstay in the characterization and staging of renal masses. When one is specifically asked to characterize a renal mass with CT, the examination needs to be performed before and after administration of intravenous contrast material [5, 13, 16].

Pre-contrast images are used for baseline density measurements in both solid and complex cystic masses but also provide information about morphological and structural features, such as the presence of intralesional fat or calcium.

Post-contrast images are used to evaluate enhancement, defined as delta of CT density measured on pre- and post-contrast images. Although contrast-enhancement do not necessarily indicate malignancy of a renal mass, either solid (i.e., oncocytoma and angiomyolipoma with no macroscopic fat) either complex cystic (i.e., inflammatory cyst), it represents the diagnostic clue that permits to differentiate surgery-needing from non-surgery-needing renal masses.

After intravenous contrast material administration, three different phases of renal enhancement are acquired: the corticomedullary phase, in which there is corticomedullary differentiation and which occurs 20–70 s after the injection of contrast material; the nephrographic phase, in which there is a homogeneous maximally enhanced nephrogram and which occurs 80–120 s after the injection of contrast material; the excretory phase, in which the nephrogram is homogeneous, but there is decreased enhancement of the parenchyma because contrast material has been excreted into the collecting system and which occurs 5-10 min after the administration of contrast material (Fig. 3.1) [17, 18].

The nephrographic phase is ideal to detect and characterize renal masses. Because there is maximal and homogeneous parenchymal enhancement, this allows better detection of renal masses, which typically do not enhance to the same degree as the renal parenchyma. The corticomedullary and excretory phases occasionally may make the detection of renal masses more difficult. During the corticomedullary phase, small renal masses may be indistinguishable from the renal medulla; during the excretory phase, renal masses may be of the same attenuation as the renal parenchyma, which has de-enhanced [17, 18].

During the nephrographic phase, most of renal neoplasms are maximally enhanced. Because the identification of enhancement is necessary to diagnose a neoplasm, maximal enhancement of neoplasms is especially important in the hypovascular ones (papillary RCC). In these cases, the unequivocal demonstration of enhancement is challenging because papillary renal cell carcinomas are particularly prone to enhance less and at a slower rate than other tumors [19]. Imaging in the corticomedullary phase is useful in showing the normal corticomedullary pattern in pseudotumors such as prominent columns of Bertin, as in depicting renal vasculature for a nephron-sparing surgery [20]. Furthermore, the typical hypervascular pattern of clear cell renal carcinoma can be demonstrated in this phase. Imaging in the excretory phases is required when an involvement of the excretory system is suspected [16–19].

When encountering any renal mass, it is necessary to first determine whether the detected abnormality represents a pseudotumor, a mass-like finding that mimics a neoplasm. Renal pseudotumors are caused by a variety of conditions including congenital anomalies (prominent renal column of Bertin, dromedary humps) (Fig. 3.2), inflammatory masses (focal pyelonephritis, chronic renal abscess, and autoimmune disease), vascular structures (renal artery aneurysm or arteriovenous fistula), or abnormalities relating to trauma or hemorrhage. Although some renal pseudotumors require treatment, they are treated differently from neoplasms, and therefore, their recognition is important to ensure proper management. If they are not first excluded when evaluating a renal mass-like finding, the application of image-based criteria used to evaluate renal masses could lead to an incorrect diagnosis. For example, enhancement is often used to support the diagnosis of a neoplasm, but enhancement can be found in infectious and other inflammatory conditions, aneurysms, and vascular malformations. Key radiologic features to support an inflammatory cause include ill-defined margins and perinephric fat stranding. Aneurysms and vascular malformations enhance similar to nearby vasculature; in the case of a vascular malformation, hypertrophy of the ipsilateral renal artery and arteriovenous shunting may be present. Finally, in the presence of a renal pseudotumor, the same degree of enhancement respect to normal parenchyma in all study phases is the diagnostic clue [20].

Once pseudotumors are excluded, mass enhancement indicates a neoplasm (Fig. 3.3). Renal mass enhancement is affected by multiple factors: the amount and rate of the contrast material injected, scan delay, and the vascularity of the mass. Highly vascular tumors show marked enhancement, whereas hypovascular tumors show minimal enhancement. There is no universally agreed upon specific value that can be used as a cutoff point for differentiating non-enhancing cysts from enhancing solid tumors. In MDCT, a threshold of 20 HU is worldwide used to indicate definitive enhancement within a renal mass, values of 10–19 HU as equivocal for enhancement, and values of less than 10 HU as indicating no enhancement [5, 20].

Cystic lesions are the most commonly detected renal masses, with most being benign simple cysts. Simple cysts are defined as having a hairline-thin wall, no septa or calcification, and being filled with simple fluid that measures 0–20 HU (Fig. 3.4). There are no soft-tissue components within simple cysts; they do not enhance after the administration of contrast medium, and they are considered benign [21–23]. When a cystic renal mass contains material showing attenuation higher than simple fluid (>20 HU), one or more septa, calcifications, thickened walls or septa, or enhancing soft-tissue components, it cannot be considered a simple cyst. The Bosniak classification system has been used worldwide in evaluating cystic renal masses over the past 25 years. Cystic renal masses are classified into 5 groups based on CT findings: categories I, II, IIF, III, and IV [22, 24–26]. Category I masses are simple cysts and are always benign. Category II masses are minimally complicated cysts that can be reliably considered as benign. They may contain a few (generally 1 or 2) hairline-thin septa in which perceived (not measurable) enhancement may be appreciated. The wall or septa may contain fine calcifications or a short segment of slightly thickened smooth calcification. High-attenuation cysts ≤3 cm are also included in category II (Fig. 3.5). These masses were initially described as containing attenuation higher than renal parenchyma (typically 40–90 HU), but the attenuation criterion has been expanded to include masses with attenuations more than 20 HU. To diagnose a high-attenuation cyst, the mass must be well circumscribed, homogeneously high-attenuating and must not enhance. Homogeneity is an important feature; high-attenuation fluid within a renal mass may mask small regions of enhancement. Therefore, it is important to obtain multiple attenuation measurements of varying sizes throughout the lesion to ensure that the mass is homogeneous and that no portion of the mass enhances [20, 22, 25, 27].

Category IIF lesions are likely benign but require follow-up imaging to show stability. These masses may contain multiple hairline-thin septa in which enhancement may be visually perceived (not measurable enhancement). Category IIF renal masses may contain thick or nodular calcification, with no soft tissue enhancing nodules. Non-enhancing high-attenuation renal cysts that measure more than 3 cm are included. Category IIF masses should be followed for morphologic and structural changes, such as development of septa, wall thickening, or new areas of enhancement, suggestive of malignancy. Growth is not a useful determinant of malignancy, because simple benign cysts may grow and renal cell carcinomas may not. Therefore, growth is not a feature of the Bosniak renal cyst classification [22–27]. The recommended interval for follow-up examinations is to obtain a CT scan or MR imaging examination at 6 and 12 months, followed by yearly examinations for a minimum of 5 years. However, there is no known time interval of stability that can be used to diagnose a renal mass as benign with complete certainty. However, whether a Bosniak category IIF lesion has not significantly morphologically changed in a period of 5 years, it is likely benign [22].

Category III cysts are truly indeterminate masses because they have a reasonable probability of being benign or malignant. Imaging features include a thickened wall or septa that show measurable enhancement. Benign masses in this category include acute and chronically infected cysts, hemorrhagic cysts (often secondary to trauma), benign multiseptated cysts, and cystic neoplasms such as multilocular cystic nephroma. Malignant masses in this category include multilocular cystic RCC. Initially, it was estimated that approximately half of category III masses were benign and the other half were malignant. Recent studies have shown a wide range (31–100 %) of these masses to be malignant. This wide variation may be attributed either to radiologists’ experience in renal mass imaging or to practice of referring urologists caring for the patient. Since category IIF was introduced, most category III renal masses that were previously removed are now being followed (category IIF), with a subsequent greater percentage of malignant category III masses remaining. Surgery remains the preferred treatment if the patient does not have high surgical risk [5, 22, 25].

Finally, category IV cysts are clearly malignant masses until proved otherwise and therefore require surgical removal. Imaging features include those described in category III with the additional presence of nodular enhancement within the mass or adjacent to its wall (Fig. 3.6). The probability of such a mass being malignant is close to 100 % [22–30].

Although there are well-established, time-tested, image-based criteria, it is often very difficult distinguishing benign complicated from malignant cystic renal masses mainly due to the overlap between the gross morphologic and radiologic findings. Thus, characterization of complex cystic renal masses, as malignant lesions requiring surgery and non-malignant lesions that do not, remains a common diagnostic challenge [28–30].

A solid renal mass is best defined as having little or no fluid components and usually consists predominantly of enhancing soft-tissue. As detailed earlier, after excluding pseudotumors, a solid renal mass should be considered a renal neoplasm. In particular, whether a solid renal mass enhances by more than 20 HU, or whether it shows enhancement in the simultaneous presence of calcification and macroscopic fat, it has to be considered as RCC until proven otherwise and pathology is mandatory (Fig. 3.7) [5, 17, 20]. It is also necessary to diagnose those malignant renal neoplasms that do not require surgery (e.g., lymphoma and metastatic disease). A combination of clinical history and imaging findings may allow these masses to be diagnosed. However, in some cases, percutaneous biopsy may be required. When there is a history of an extra-renal primary tumor, the 50–85 % of solitary renal masses are metastatic. Therefore, if a solid renal mass is detected in a patient with a known primary malignancy (e.g., lung cancer, lymphoma), a metastasis should not be necessarily diagnosed presumptively; both a second primary (RCC) and a benign neoplasm should be considered [5, 17, 20].

However, many small (≤3 cm) solid renal masses are benign [31]. Benign diagnoses typically encountered at surgery, for what was previously believed to be RCC mainly include oncocytomas and angiomyolipomas.

Oncocytoma is a benign solid renal mass that cannot be differentiated from RCC by imaging. Although a central scar or homogeneous enhancement at CT or MRI is suggestive of oncocytoma, these findings are not specific and a tissue diagnosis is necessary to differentiate oncocytoma from RCC [31–33].

Angiomyolipoma is a benign neoplasm that, particularly when small, warrants no treatment. Most angiomyolipomas can be diagnosed by identifying regions of fat within a non-calcified renal mass at unenhanced CT. Some angiomyolipomas contain very small quantities of fat that can be overlooked if the mass is not carefully evaluated. However, approximately 5 % of angiomyolipomas do not contain fat that can be seen at imaging, and the differentiation from other renal neoplasms is not possible with CT. These masses, referred to as angiomyolipoma with minimal fat, are often small, mildly hyperattenuating on unenhanced CT examination and homogeneously enhanced with contrast material. However, these findings are not specific enough to make a confident diagnosis of an angiomyolipoma, and other tumors, such as papillary RCC, metanephric adenoma, oncocytoma and leiomyoma, may have a similar appearance [34, 35].

Further problems in enhancement evaluation, mainly due to multiphasic acquisition, i.e., different positioning of region-of-interest (ROI) on sequential images or difficulty with appropriate placement of a ROI in intrarenal tumors that are isodense to parenchyma on unenhanced image, make sometimes a definitive diagnosis difficult at MDCT, especially when small hypovascular tumors are encountered. Another frequent problem is represented by the pseudoenhancement, which is defined as an artificial elevation of the HU attenuation of a simple renal cyst imaged on contrast-enhanced CT image during nephrographic phase, at peak of parenchymal enhancement. Pseudoenhancement is thought to be principally due to the image reconstruction algorithms used to adjust for beam-hardening artifacts [36, 37].

By means of spectral analysis, dual-energy imaging provide complementary information and approach toward a new hybrid, morphologic-functional imaging.

Various clinically relevant applications have been established for dual-energy CT renal lesion imaging [38–45]. These applications can be distinguished in non-material–specific, material–specific, and energy–specific (Fig. 3.8).